Health officials in the Republic are investigating the outbreak of a potentially deadly strain of e-coli.
A child is understood to be among a number of people who have suffered from severe vomiting and diarrhoea.
It's believed most of the cases are centred around Inch Island, around seven miles from Buncrana in Co Donegal.
The illness is believed to be the 0157 strain, which has been associated with eating undercooked, contaminated beef, swimming in or drinking contaminated water, or eating contaminated vegetables. The public water supply, however, has been given the all-clear by the authorities.
In extreme cases, it can lead to liver failure in young children and the elderly.
The Health and Safety Executive has confirmed there has been an outbreak of e-coli, but won't say how many people are affected because of patient confidentiality. The source of the illness has not been traced.
It says the Department of Public Health is investigating the outbreak, but all tests taken so far have ruled out the public water supply as the cause.
A spokeswoman for HSE West said: “In the majority of cases, this is a self-limiting illness, the patients get better by themselves without further treatment.”